There are probably many factors involved why they do not hire them.
Just a few I can think of.
1) established pay rate for Fed employees [they would only be hired part time to equal their unemployment rate.]
2) Overhead cost for the employee. [ which would put the weekly cost of the unemployed person higher than what they are getting on unemployment]
3) Taxes with held on the employee [ The unemployed person would be paying taxes on the wages ] then the employee would need to work longer hours to pay taxes and have the same left over as they are getting on unemployment. More pay would increase overhead cost to the feds.

4) the employee would be paying transportation cost to work which would cut into his take home check.
5) Then the unemployed receiving benefits would be considered employed which would entitle him to unemployment benefits if funds ran out for this idea.
6) In a nutshell it will cost the states less money to just pay unemployment till it runs out.